The invention relates to a device for optically measuring the angular velocity of a rotating object, which device comprises:
a rotatable first disc adapted to be driven by the object, and a second disc, both discs being provided with a periodical and rotationally symmetrical pattern of elongated interruptions in a disc face; PA0 an illumination system for simultaneously illuminating the complete pattern; and PA0 a detection system. PA0 a reproducing head for reproducing an information signal recorded on the tape, PA0 a buffer memory, an input of which is coupled to an output of the reproducing head and an output of which is coupled to an output terminal for supplying the information signal, PA0 first clock signal generator means for generating and supplying a first clock signal having a first clock frequency at an output which is coupled to a write clock signal input of the buffer memory, said buffer memory being adapted to store the information signal in response to the first clock signal, PA0 second clock signal generator means for generating and supplying a second clock signal having a second clock frequency at an output which is coupled to a read clock signal input of the buffer memory, said buffer memory being adapted to supply the information signal stored in the buffer memory at the output in response to the second clock signal, one of the first or the second clock signal generator means being adapted to generate a clock signal having a fixed clock frequency, PA0 detection means for determining a filling level of the buffer memory and for supplying a control signal at an output, which control signal is a measure of the filling level of the buffer memory, is further characterized in that an output of the speed sensor is coupled to an input of the other one of the first or the second clock signal generator means, in that said other clock signal generator means are adapted to derive a clock signal having a clock frequency in dependence upon the electric signal of the device, and in that the output of the detection means is coupled to an input of the control circuit for controlling the speed of the take-up reel.
The invention also relates to a device of this type which is made suitable for measuring the linear velocity of an elongated object.
The invention further relates to a capstanless tape scanning apparatus provided with two reels to be used as supply reel and take-up reel, a scanning head arranged in the path covered by the tape between the two reels and a tape speed control loop comprising a tape speed sensor and a control circuit for controlling the speed of a motor for a reel functioning as the take-up reel.
An optical angular velocity measuring device is known from U.S. Pat. No. 4,658,132.
Capstanless tape scanning apparatuses are known in various embodiments and described in, inter alia GB Patent Specification 1,330,923 and U.S. Pat. No. 3,809,335. As has been noted in the GB Patent Specification, the use of a capstan in combination with a pressure mechanism for driving a tape at a constant linear speed has the advantage that an accurate control of the tape speed can be realised at minimal cost as far as the control itself is concerned. However, when a capstan is used, a number of drawbacks occur, three of which have been mentioned in the British Patent Specification, viz. a complicated mechanism for introducing the cassette, difficult guidance of the edges of the tape and a greater risk of damage of the tape. Moreover, there are problems of contamination and a slipping tape. Said drawbacks and problems can be prevented by driving the tape in a different manner than with the aid of a capstan, viz. by means of the supply reel and/or the take-up reel. In this case, provisions have to be made to keep the tape speed constant at the location of the scanning head. A possible provision mentioned in the GB Patent Specification GB 1,330,923 is the use of a tape-driven tachometer which is not further described and, moreover, is discarded as being less suitable. In the GB Patent Specification a choice is made for a system in which one of the reels is driven by means of a control signal and a non-linear signal which is superimposed thereon and is inversely proportional to the diameter of the tape on the relevant reel. In a capstanless tape scanning apparatus in which the tape speed is controlled by the driving mechanism of the take-up reel, a slip coupling should be present in this driving mechanism, which slip coupling leads to loss of energy. U.S. Pat. No. 3,809,335 describes a tape driving apparatus in which the supply reel and the take-up reel are driven and in which acceleration, deceleration, speed, as well as tape tension are measured. This apparatus does not comprise a separate tape speed measuring station.
Since the scanning head of a tape player is present at some distance from both the supply reel and the take-up reel, it is preferred to use a separate speed measuring station which is arranged close to the scanning head so that the speed is measured at the location where the speed should be constant. The signal of the speed measuring station may alternatively be used as a time base correction when processing the information signal read from the tape. It is further preferred to use an optical measuring device because such a device can be implemented at low cost and at small dimensions by means of the present-day techniques and because it has a high resolving power and a great measuring accuracy. As compared with a magnetic angular velocity sensor, the optical sensor has the advantages that the quality of the measured output signal is less dependent on the range of revolution frequencies in which the measurements take place and that it is less sensitive to electric interference. In a magnetic sensor, disturbing interactions between the magnet and the bearings may occur.
The optical angular velocity measuring device described in U.S. Pat. No. 4,658,132 and intended for measuring the angular velocity of a motor could be considered for use in a tape speed measuring station. The device described in U.S. Pat. No. 4,658,132 comprises a first disc rotating with the motor shaft, on which disc interruptions in the form of light-transmissive slits are arranged in the form of a ring, and a second stationary disc which is provided with a plurality of radiation-sensitive detection elements also arranged in a ring, the number of detection elements corresponding to the number of slits on the rotatable disc. This disc is illuminated and the radiation passing through the slits is received by the detection elements. When the motor is rotated, the slits are displaced with respect to the detection elements so that maximum and minimum quantities of radiation are alternately received by the detection elements. The sum of the output signals of these elements is a periodical signal whose frequency represents the angular velocity of the slotted disc and hence that of the motor.
German Patent Application 2 155970 describes another angular velocity sensor comprising a circularly bent tubular radiation source within which a rotating slotted drum and a stationary slotted drum are arranged, and in which a bundle of optical fibers transports the radiation passing through the two slotted drums to a detector.